Sensing against a reference cell

ABSTRACT

Memory devices, bulk storage devices, and methods of operating memory are disclosed, such as those adapted to process and generate analog data signals representative of data values of two or more bits of information. Programming of such memory devices can include programming to a target threshold voltage within a range representative of the desired bit pattern. Reading such memory devices can include generating an analog data signal indicative of a threshold voltage of a target memory cell. The target memory cell can be sensed against a reference cell includes a dummy string of memory cells connected to a target string of memory cells, and, such as by using a differential amplifier to sense a difference between a reference cell and the target cell. This may allow a wider range of voltages to be used for data states.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/137,988, titled “SENSING AGAINST A REFERENCE CELL,” filed Jun. 12,2008, (allowed) which is commonly assigned and incorporated herein byreference.

FIELD

The present disclosure relates generally to semiconductor memory, and inparticular, the present disclosure relates to solid state non-volatilememory devices and systems utilizing analog signals to communicate datavalues of two or more bits of information.

BACKGROUND

Electronic devices commonly have some type of bulk storage deviceavailable to them. A common example is a hard disk drive (HDD). HDDs arecapable of large amounts of storage at relatively low cost, with currentconsumer HDDs available with over one terabyte of capacity.

HDDs generally store data on rotating magnetic media or platters. Datais typically stored as a pattern of magnetic flux reversals on theplatters. To write data to a typical HDD, the platter is rotated at highspeed while a write head floating above the platter generates a seriesof magnetic pulses to align magnetic particles on the platter torepresent the data. To read data from a typical HDD, resistance changesare induced in a magnetoresistive read head as it floats above theplatter rotated at high speed. In practice, the resulting data signal isan analog signal whose peaks and valleys are the result of the magneticflux reversals of the data pattern. Digital signal processing techniquescalled partial response maximum likelihood (PRML) are then used tosample the analog data signal to determine the likely data patternresponsible for generating the data signal.

HDDs have certain drawbacks due to their mechanical nature. HDDs aresusceptible to damage or excessive read/write errors due to shock,vibration or strong magnetic fields. In addition, they are relativelylarge users of power in portable electronic devices.

Another example of a bulk storage device is a solid state drive (SSD).Instead of storing data on rotating media, SSDs utilize semiconductormemory devices to store their data, but include an interface and formfactor making them appear to their host system as if they are a typicalHDD. The memory devices of SSDs are typically non-volatile flash memorydevices.

Flash memory devices have developed into a popular source ofnon-volatile memory for a wide range of electronic applications. Flashmemory devices typically use a one-transistor memory cell that allowsfor high memory densities, high reliability, and low power consumption.Changes in threshold voltage of the cells, through programming of chargestorage or trapping layers or other physical phenomena, determine thedata value of each cell. Common uses for flash memory and othernon-volatile memory include personal computers, personal digitalassistants (PDAs), digital cameras, digital media players, digitalrecorders, games, appliances, vehicles, wireless devices, mobiletelephones, and removable memory modules, and the uses for non-volatilememory continue to expand.

Unlike HDDs, the operation of SSDs is generally not subject tovibration, shock or magnetic field concerns due to their solid statenature. Similarly, without moving parts, SSDs have lower powerrequirements than HDDs. However, SSDs currently have much lower storagecapacities compared to HDDs of the same form factor and a significantlyhigher cost per bit.

For the reasons stated above, and for other reasons which will becomeapparent to those skilled in the art upon reading and understanding thepresent specification, there is a need in the art for alternative bulkstorage options.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a simplified block diagram of a memory device according to anembodiment of the disclosure.

FIG. 2 is a block schematic of a solid state bulk storage device inaccordance with one embodiment of the present disclosure.

FIG. 3 is a depiction of a wave form showing conceptually a data signalas might be received from the memory device by a read/write channel inaccordance with an embodiment of the disclosure.

FIG. 4 is a block schematic of an electronic system in accordance withan embodiment of the disclosure.

FIG. 5 is a circuit diagram of a differential amplifier in accordancewith another embodiment of the present invention.

FIG. 6 is a circuit diagram of a differential amplifier in conjunctionwith a memory array according to another embodiment of the presentinvention.

DETAILED DESCRIPTION

In the following detailed description of the present embodiments,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration specific embodiments inwhich the embodiments may be practiced. These embodiments are describedin sufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that process, electrical or mechanical changes may be madewithout departing from the scope of the present disclosure. Thefollowing detailed description is, therefore, not to be taken in alimiting sense.

Traditional solid-state memory devices pass data in the form of binarysignals. Typically, a ground potential represents a first logic level ofa bit of data, e.g., a ‘0’ data value, while a supply potentialrepresents a second logic level of a bit of data, e.g., a ‘1’ datavalue. A multi-level cell (MLC) may be assigned, for example, fourdifferent threshold voltage (Vt) ranges of 200 mV for each range, witheach range corresponding to a distinct data state, thereby representingfour data values or bit patterns. Typically, a dead space or margin of0.2V to 0.4V is between each range to keep the Vt distributions fromoverlapping. If the Vt of the cell is within the first range, the cellmay be deemed to store a logical 11 state and is typically consideredthe erased state of the cell. If the Vt is within the second range, thecell may be deemed to store a logical 10 state. If the Vt is within thethird range, the cell may be deemed to store a logical 00 state. And ifthe Vt is within the fourth range, the cell may be deemed to store alogical 01 state.

When programming a traditional MLC device as described above, cells aregenerally first erased, as a block, to correspond to the erased state.Following erasure of a block of cells, the least-significant bit (LSB)of each cell is first programmed, if necessary. For example, if the LSBis a 1, then no programming is necessary, but if the LSB is a 0, thenthe Vt of the target memory cell is moved from the Vt rangecorresponding to the 11 logic state to the Vt range corresponding to the10 logic state. Following programming of the LSBs, the most-significantbit (MSB) of each cell is programmed in a similar manner, shifting theVt where necessary. When reading an MLC of a traditional memory device,one or more read operations determine generally into which of the rangesthe Vt of the cell voltage falls. For example, a first read operationmay determine whether the Vt of the target memory cell is indicative ofthe MSB being a 1 or a 0 while a second read operation may determinewhether the Vt of the target memory cell in indicative of the LSB beinga 1 or a 0. In each case, however, a single bit is returned from a readoperation of a target memory cell, regardless of how many bits arestored on each cell. This problem of multiple program and readoperations becomes increasingly troublesome as more bits are stored oneach MLC. Because each such program or read operation is a binaryoperation, i.e., each programs or returns a single bit of informationper cell, storing more bits on each MLC leads to longer operation times.

The memory devices of an illustrative embodiment store data as Vt rangeson the memory cells. In contrast to traditional memory devices, however,program and read operations are capable of utilizing data signals not asdiscrete bits of MLC data values, but as full representations of MLCdata values, such as their complete bit patterns. For example, in atwo-bit MLC device, instead of programming a cell's LSB and subsequentlyprogramming that cell's MSB, a target threshold voltage may beprogrammed representing the bit pattern of those two bits. That is, aseries of program and verify operations would be applied to a memorycell until that memory cell obtained its target threshold voltage ratherthan programming to a first threshold voltage for a first bit, shiftingto a second threshold voltage for a second bit, etc. Similarly, insteadof utilizing multiple read operations to determine each bit stored on acell, the threshold voltage of the cell may be determined and passed asa single signal representing the complete data value or bit pattern ofthe cell. The memory devices of the various embodiments do not merelylook to whether a memory cell has a threshold voltage above or belowsome nominal threshold voltage as is done in traditional memory devices.Instead, a voltage signal is generated that is representative of theactual threshold voltage of that memory cell across the continuum ofpossible threshold voltages. An advantage of this approach becomes moresignificant as the bits per cell count is increased. For example, if thememory cell were to store eight bits of information, a single readoperation would return a single analog data signal representative ofeight bits of information.

FIG. 1 is a simplified block diagram of a memory device 101 according toan embodiment of the disclosure. Memory device 101 includes an array ofmemory cells 104 arranged in rows and columns. Although the variousembodiments will be described primarily with reference to NAND memoryarrays, the various embodiments are not limited to a specificarchitecture of the memory array 104. Some examples of other arrayarchitectures suitable for the present embodiments include NOR arrays,AND arrays, and virtual ground arrays. In general, however, theembodiments described herein are adaptable to any array architecturepermitting generation of a data signal indicative of the thresholdvoltage of each memory cell.

A row decode circuitry 108 and a column decode circuitry 110 areprovided to decode address signals provided to the memory device 101.Address signals are received and decoded to access memory array 104.Memory device 101 also includes input/output (I/O) control circuitry 112to manage input of commands, addresses and data to the memory device 101as well as output of data and status information from the memory device101. An address register 114 is coupled between I/O control circuitry112 and row decode circuitry 108 and column decode circuitry 110 tolatch the address signals prior to decoding. A command register 124 iscoupled between I/O control circuitry 112 and control logic 116 to latchincoming commands. Control logic 116 controls access to the memory array104 in response to the commands and generates status information for theexternal processor 130. The control logic 116 is coupled to row decodecircuitry 108 and column decode circuitry 110 to control the row decodecircuitry 108 and column decode circuitry 110 in response to theaddresses.

Control logic 116 is also coupled to a sample and hold circuitry 118.The sample and hold circuitry 118 latches data, either incoming oroutgoing, in the form of analog voltage levels. For example, the sampleand hold circuitry could contain capacitors or other analog storagedevices for sampling either an incoming voltage signal representing datato be written to a memory cell or an outgoing voltage signal indicativeof the threshold voltage sensed from a memory cell. The sample and holdcircuitry 118 may further provide for amplification and/or buffering ofthe sampled voltage to provide a stronger data signal to an externaldevice.

The handling of analog voltage signals may take an approach similar toan approach well known in the area of CMOS imager technology, wherecharge levels generated at pixels of the imager in response to incidentillumination are stored on capacitors. These charge levels are thenconverted to voltage signals using a differential amplifier with areference capacitor as a second input to the differential amplifier. Theoutput of the differential amplifier is then passed to analog-to-digitalconversion (ADC) devices to obtain a digital value representative of anintensity of the illumination. In the present embodiments, a charge maybe stored on a capacitor in response to subjecting it to a voltage levelindicative of an actual or target threshold voltage of a memory cell forreading or programming, respectively, the memory cell. This charge couldthen be converted to an analog voltage using a differential amplifierhaving a grounded input or other reference signal as a second input. Theoutput of the differential amplifier could then be passed to the I/Ocontrol circuitry 112 for output from the memory device, in the case ofa read operation, or used for comparison during one or more verifyoperations in programming the memory device. It is noted that the I/Ocontrol circuitry 112 could optionally include analog-to-digitalconversion functionality and digital-to-analog conversion (DAC)functionality to convert read data from an analog signal to a digitalbit pattern and to convert write data from a digital bit pattern to ananalog signal such that the memory device 101 could be adapted forcommunication with either an analog or digital data interface.

During a write operation, target memory cells of the memory array 104are programmed until voltages indicative of their Vt levels match thelevels held in the sample and hold circuitry 118. This can beaccomplished, as one example, using differential sensing devices tocompare the held voltage level to a threshold voltage of the targetmemory cell. Much like traditional memory programming, programmingpulses could be applied to a target memory cell to increase itsthreshold voltage until reaching or exceeding the desired value. In aread operation, the Vt levels of the target memory cells are passed tothe sample and hold circuitry 118 for transfer to an external processor(not shown in FIG. 1) either directly as analog signals or as digitizedrepresentations of the analog signals depending upon whether ADC/DACfunctionality is provided external to, or within, the memory device.

Threshold voltages of cells may be determined in a variety of manners.For example, a word line voltage could be sampled at the point when thetarget memory cell becomes activated. Alternatively, a boosted voltagecould be applied to a first source/drain side of a target memory cell,and the threshold voltage could be taken as a difference between itscontrol gate voltage and the voltage at its other source/drain side. Bycoupling the voltage to a capacitor, charge would be shared with thecapacitor to store the sampled voltage. Note that the sampled voltageneed not be equal to the threshold voltage, but merely indicative ofthat voltage. For example, in the case of applying a boosted voltage toa first source/drain side of the memory cell and a known voltage to itscontrol gate, the voltage developed at the second source/drain side ofthe memory cell could be taken as the data signal as the developedvoltage is indicative of the threshold voltage of the memory cell.

Sample and hold circuitry 118 may include caching, i.e., multiplestorage locations for each data value, such that the memory device 101may be reading a next data value while passing a first data value to theexternal processor, or receiving a next data value while writing a firstdata value to the memory array 104. A status register 122 is coupledbetween I/O control circuitry 112 and control logic 116 to latch thestatus information for output to the external processor.

Memory device 101 receives control signals at control logic 116 over acontrol link 132. The control signals may include a chip enable CE#, acommand latch enable CLE, an address latch enable ALE, and a writeenable WE#. Memory device 101 may receive commands (in the form ofcommand signals), addresses (in the form of address signals), and data(in the form of data signals) from an external processor over amultiplexed input/output (I/O) bus 134 and output data to the externalprocessor over I/O bus 134.

In a specific example, commands are received over input/output (I/O)pins [7:0] of I/O bus 134 at I/O control circuitry 112 and are writteninto command register 124. The addresses are received over input/output(I/O) pins [7:0] of bus 134 at I/O control circuitry 112 and are writteninto address register 114. The data may be received over input/output(I/O) pins [7:0] for a device capable of receiving eight parallelsignals, or input/output (I/O) pins [15:0] for a device capable ofreceiving sixteen parallel signals, at I/O control circuitry 112 and aretransferred to sample and hold circuitry 118. Data also may be outputover input/output (I/O) pins [7:0] for a device capable of transmittingeight parallel signals or input/output (I/O) pins [15:0] for a devicecapable of transmitting sixteen parallel signals. It will be appreciatedby those skilled in the art that additional circuitry and signals can beprovided, and that the memory device of FIG. 1 has been simplified tohelp focus on the embodiments of the disclosure. Additionally, while thememory device of FIG. 1 has been described in accordance with popularconventions for receipt and output of the various signals, it is notedthat the various embodiments are not limited by the specific signals andI/O configurations described unless expressly noted herein. For example,command and address signals could be received at inputs separate fromthose receiving the data signals, or data signals could be transmittedserially over a single I/O line of I/O bus 134. Because the data signalsrepresent bit patterns instead of individual bits, serial communicationof an 8-bit data signal could be as efficient as parallel communicationof eight signals representing individual bits.

Memory devices of the various embodiments may be advantageously used inbulk storage devices. For various embodiments, these bulk storagedevices may take on the same form factor and communication bus interfaceof traditional HDDs, thus allowing them to replace such drives in avariety of applications. Some common form factors for HDDs include the3.5″, 2.5″ and PCMCIA (Personal Computer Memory Card InternationalAssociation) form factors commonly used with current personal computersand larger digital media recorders, as well as 1.8″ and 1″ form factorscommonly used in smaller personal appliances, such as mobile telephones,personal digital assistants (PDAs) and digital media players. Somecommon bus interfaces include universal serial bus (USB), AT attachmentinterface (ATA) [also known as integrated drive electronics or IDE],serial ATA (SATA), small computer systems interface (SCSI) and theInstitute of Electrical and Electronics Engineers (IEEE) 1394 standard.While a variety of form factors and communication interfaces werelisted, the embodiments are not limited to a specific form factor orcommunication standard. Furthermore, the embodiments need not conform toa HDD form factor or communication interface. FIG. 2 is a blockschematic of a solid state bulk storage device 300 in accordance withone embodiment of the present disclosure.

The bulk storage device 300 includes a memory device 301 in accordancewith an embodiment of the disclosure, a read/write channel 305 and acontroller 310. The read/write channel 305 provides foranalog-to-digital conversion of data signals received from the memorydevice 301 as well as digital-to-analog conversion of data signalsreceived from the controller 310. The controller 310 provides forcommunication between the bulk storage device 300 and an externalprocessor (not shown in FIG. 2) through bus interface 315. It is notedthat the read/write channel 305 could service one or more additionalmemory devices, as depicted by memory device 301′ in dashed lines.Selection of a single memory device 301 for communication can be handledthrough a multi-bit chip enable signal or other multiplexing scheme.

The memory device 301 is coupled to a read/write channel 305 through ananalog interface 320 and a digital interface 325. The analog interface320 provides for the passage of analog data signals between the memorydevice 301 and the read/write channel 305 while the digital interface325 provides for the passage of control signals, command signals andaddress signals from the read/write channel 305 to the memory device301. The digital interface 325 may further provide for the passage ofstatus signals from the memory device 301 to the read/write channel 305.The analog interface 320 and the digital interface 325 may share signallines as noted with respect to the memory device 101 of FIG. 1. Althoughthe embodiment of FIG. 2 depicts a dual analog/digital interface to thememory device, functionality of the read/write channel 305 couldoptionally be incorporated into the memory device 301 as discussed withrespect to FIG. 1 such that the memory device 301 communicates directlywith the controller 310 using only a digital interface for passage ofcontrol signals, command signals, status signals, address signals anddata signals.

The read/write channel 305 is coupled to the controller 310 through oneor more interfaces, such as a data interface 330 and a control interface335. The data interface 330 provides for the passage of digital datasignals between the read/write channel 305 and the controller 310. Thecontrol interface 335 provides for the passage of control signals,command signals and address signals from the controller 310 to theread/write channel 305. The control interface 335 may further providefor the passage of status signals from the read/write channel 305 to thecontroller 310. Status and command/control signals may also be passeddirectly between the controller 310 and the memory device 301 asdepicted by the dashed line connecting the control interface 335 to thedigital interface 325.

Although depicted as two distinct devices in FIG. 2, the functionalityof the read/write channel 305 and the controller 310 could alternativelybe performed by a single integrated circuit device. And whilemaintaining the memory device 301 as a separate device would providemore flexibility in adapting the embodiments to different form factorsand communication interfaces, because it is also an integrated circuitdevice, the entire bulk storage device 300 could be fabricated as asingle integrated circuit device.

The read/write channel 305 is a signal processor adapted to at leastprovide for conversion of a digital data stream to an analog data streamand vice versa. A digital data stream provides data signals in the formof binary voltage levels, i.e., a first voltage level indicative of abit having a first binary data value, e.g., 0, and a second voltagelevel indicative of a bit having a second binary data value, e.g., 1. Ananalog data stream provides data signals in the form of analog voltageshaving more than two levels, with different voltage levels or rangescorresponding to different bit patterns of two or more bits. Forexample, in a system adapted to store two bits per memory cell, a firstvoltage level or range of voltage levels of an analog data stream couldcorrespond to a bit pattern of 11, a second voltage level or range ofvoltage levels of an analog data stream could correspond to a bitpattern of 10, a third voltage level or range of voltage levels of ananalog data stream could correspond to a bit pattern of 00 and a fourthvoltage level or range of voltage levels of an analog data stream couldcorrespond to a bit pattern of 01. Thus, one analog data signal inaccordance with the various embodiments would be converted to two ormore digital data signals, and vice versa.

In practice, control and command signals are received at the businterface 315 for access of the memory device 301 through the controller310. Addresses and data values may also be received at the bus interface315 depending upon what type of access is desired, e.g., write, read,format, etc. In a shared bus system, the bus interface 315 would becoupled to a bus along with a variety of other devices. To directcommunications to a specific device, an identification value may beplaced on the bus indicating which device on the bus is to act upon asubsequent command. If the identification value matches the value takenon by the bulk storage device 300, the controller 310 would then acceptthe subsequent command at the bus interface 315. If the identificationvalue did not match, the controller 310 would ignore the subsequentcommunication. Similarly, to avoid collisions on the bus, the variousdevices on a shared bus may instruct other devices to cease outboundcommunication while they individually take control of the bus. Protocolsfor bus sharing and collision avoidance are well known and will not bedetailed herein. The controller 310 then passes the command, address anddata signals on to the read/write channel 305 for processing. Note thatthe command, address and data signals passed from the controller 310 tothe read/write channel 305 need not be the same signals received at thebus interface 315. For example, the communication standard for the businterface 315 may differ from the communication standard of theread/write channel 305 or the memory device 301. In this situation, thecontroller 310 may translate the commands and/or addressing scheme priorto accessing the memory device 301. In addition, the controller 310 mayprovide for load leveling within the one or more memory devices 301,such that physical addresses of the memory devices 301 may change overtime for a given logical address. Thus, the controller 310 would map thelogical address from the external device to a physical address of atarget memory device 301.

For write requests, in addition to the command and address signals, thecontroller 310 would pass digital data signals to the read/write channel305. For example, for a 16-bit data word, the controller 310 would pass16 individual signals having a first or second binary logic level. Theread/write channel 305 would then convert the digital data signals to ananalog data signal representative of the bit pattern of the digital datasignals. To continue with the foregoing example, the read/write channel305 would use a digital-to-analog conversion to convert the 16individual digital data signals to a single analog signal having apotential level indicative of the desired 16-bit data pattern. For oneembodiment, the analog data signal representative of the bit pattern ofthe digital data signals is indicative of a desired threshold voltage ofthe target memory cell. However, in programming of a one-transistormemory cells, it is often the case that programming of neighboringmemory cells will increase the threshold voltage of previouslyprogrammed memory cells. Thus, for another embodiment, the read/writechannel 305 can take into account these types of expected changes in thethreshold voltage, and adjust the analog data signal to be indicative ofa threshold voltage lower than the final desired threshold voltage.After conversion of the digital data signals from the controller 310,the read/write channel 305 would then pass the write command and addresssignals to the memory device 301 along with the analog data signals foruse in programming the individual memory cells. Programming can occur ona cell-by-cell basis, but is generally performed for a page of data peroperation. For a typical memory array architecture, a page of dataincludes every other memory cell coupled to a word line.

For read requests, the controller would pass command and address signalsto the read/write channel 305. The read/write channel 305 would pass theread command and address signals to the memory device 301. In response,after performing the read operation, the memory device 301 would returnthe analog data signals indicative of the threshold voltages of thememory cells defined by the address signals and the read command. Thememory device 301 may transfer its analog data signals in parallel orserial fashion.

The analog data signals may also be transferred not as discrete voltagepulses, but as a substantially continuous stream of analog signals. Inthis situation, the read/write channel 305 may employ signal processingsimilar to that used in HDD accessing called PRML or partial response,maximum likelihood. In PRML processing of a traditional HDD, the readhead of the HDD outputs a stream of analog signals representative offlux reversals encountered during a read operation of the HDD platter.Rather than attempting to capture the true peaks and valleys of thisanalog signal generated in response to flux reversals encountered by theread head, the signal is periodically sampled to create a digitalrepresentation of the signal pattern. This digital representation canthen be analyzed to determine the likely pattern of flux reversalsresponsible for generation of the analog signal pattern. This same typeof processing can be utilized with embodiments of the presentdisclosure. By sampling the analog signal from the memory device 301,PRML processing can be employed to determine the likely pattern ofthreshold voltages responsible for generation of the analog signal.

FIG. 3 is a depiction of a wave form showing conceptually a data signal450 as might be received from the memory device 301 by the read/writechannel 305 in accordance with an embodiment of the disclosure. The datasignal 450 could be periodically sampled and a digital representation ofthe data signal 450 can be created from the amplitudes of the sampledvoltage levels. For one embodiment, the sampling could be synchronizedto the data output such that sampling occurs during the steady-stateportions of the data signal 450. Such an embodiment is depicted by thesampling as indicated by the dashed lines at times t1, t2, t3 and t4.However, if synchronized sampling becomes misaligned, values of the datasamples may be significantly different than the steady-state values. Inan alternate embodiment, sampling rates could be increased to allowdetermination of where steady-state values likely occurred, such as byobserving slope changes indicated by the data samples. Such anembodiment is depicted by the sampling as indicated by the dashed linesat times t5, t6, t7 and t8, where a slope between data samples at timest6 and t7 may indicate a steady-state condition. In such an embodiment,a trade-off is made between sampling rate and accuracy of therepresentation. Higher sampling rates lead to more accuraterepresentations, but also increase processing time. Regardless ofwhether sampling is synchronized to the data output or more frequentsampling is used, the digital representation can then be used to predictwhat incoming voltage levels were likely responsible for generating theanalog signal pattern. In turn, the likely data values of the individualmemory cells being read can be predicted from this expected pattern ofincoming voltage levels.

Recognizing that errors will occur in the reading of data values fromthe memory device 301, the read/write channel 305 may include errorcorrection. Error correction is commonly used in memory devices, as wellas HDDs, to recover from expected errors. Typically, a memory devicewill store user data in a first set of locations and error correctioncode (ECC) in a second set of locations. During a read operation, boththe user data and the ECC are read in response to a read request of theuser data. Using known algorithms, the user data returned from the readoperation is compared to the ECC. If the errors are within the limits ofthe ECC, the errors will be corrected.

FIG. 4 is a block schematic of an electronic system in accordance withan embodiment of the disclosure. Example electronic systems may includepersonal computers, PDAs, digital cameras, digital media players,digital recorders, electronic games, appliances, vehicles, wirelessdevices, mobile telephones and the like.

The electronic system includes a host processor 500 that may includecache memory 502 to increase the efficiency of the processor 500. Theprocessor 500 is coupled to a communication bus 504. A variety of otherdevices may be coupled to the communication bus 504 under control of theprocessor 500. For example, the electronic system may include randomaccess memory (RAM) 506; one or more input devices 508 such askeyboards, touch pads, pointing devices, etc.; an audio controller 510;a video controller 512; and one or more bulk storage devices 514. Atleast one bulk storage device 514 includes a digital bus interface 515for communication with the bus 504, one or more memory devices inaccordance with an embodiment of the disclosure having an analoginterface for transfer of data signals representative of data patternsof two or more bits of data, and a signal processor adapted to performdigital-to-analog conversion of digital data signals received from thebus interface 515 and analog-to-digital conversion of analog datasignals received from its memory device(s).

Sensing Against a Reference Cell

Temperature and process variations in memories can lead to differentcells having different characteristics. Cells behave differently astemperature changes. Also, variations in process can also result invariations in cell operation and voltages stored on the device.Typically, a voltage stored on a memory cell is an absolute voltage. Ina typical configuration, any absolute voltage between 0 volts and −3volts is the same for purposes of storing and reading. That is, anyvoltage within that range corresponds to just one data state. There isonly one state in the voltage range from 0 volts to −3 volts when usingabsolute voltage readings. Various embodiments described herein providefor a differential reading instead of reading an absolute voltage,allowing temperature and process variations to be mitigated, and openingup the 0 volts to −3 volts range to be usable for more than one datastate.

In one embodiment, a reference cell and target cell coupled with a PMOScurrent mirror load and a bias current source make up a differentialamplifier. The reference and target cells are part of this differentialamplifier. The embodiments of the present invention allow sensing of adifference between the target memory cell and the reference cell.Sensing the difference between the threshold voltages of the targetmemory cell and the reference cell mitigates temperature and processvariations and effects as both the target memory cell and the referencecell move in similar manners in response to temperature variations.Also, sensing a threshold voltage difference between the target memorycell and the reference cell allows usage of an entire voltage windowrange for sensing. In a voltage range of, for example, −3 volts to 3.5volts, the range of −3 volts to 0 volts typically corresponds to onedata state as a ground potential on the word line will activate the cellregardless of where in the range its actual threshold voltage falls.However, in using a threshold voltage difference between the targetmemory cell and the reference cell, sub-ranges within the negativevoltage portion of the threshold voltage range can correspond toadditional data states.

Further, the target cells and the reference cells are in one embodimentformed at substantially the same time, and in another embodiment areformed using substantially the same process. This makes the target cellsand the reference cells substantially similar in construction. A circuitaccording to one embodiment is shown in FIG. 5, and includes adifferential amplifier 606 having a set of PMOS transistors 610, areference string 608 with a selected reference cell 609, a selectedstring 602 with a selected target cell 603, and a bias current source612. As described above, the reference cell 609 and target cell 603 arecoupled with PMOS current mirror load 610 and a bias current source 612to make up the differential amplifier 606.

For illustration purposes, the details of the reference string 608 arenot shown. Instead, a single reference cell 609 is shown. In oneembodiment, the reference string is a string of cells, such as NANDcells. In another embodiment, the reference string has a single cell. Itshould be understood that other permutations and variations of referencestrings, including by way of example only and not by way of limitationAND strings and other architectures, are amenable to use with theembodiments described herein without departing from the scope of theembodiments.

In operation, differential amplifier 606 has a string of memory cells602 including selected cell 603, a string of reference memory cells 608including memory cell 609 is connected to a string of target cells as isshown in FIG. 6. The cell 609, which may be one in the string ofreference cells 608, is programmed to a known voltage. By programmingthe cell 609 to a known voltage, the threshold voltage differencebetween the cell 609 and a target cell can be determined using thedifferential amplifier 606. The various embodiments of differentialsensing allow for an output voltage differential from 0 to 6.5 volts,rendering the entire actual voltage range from −3 to 3.5 volts usablefor data states. For example, the reference cell 609 can be programmedto a Vt of −3V and Vsel (applied to the gate of the target cell 603) canbe biased at 6.5V. When the Vt of selected cell 603 is −3V, the outputof the differential amplifier 606 is ideally 6.5V. When the Vt ofselected cell 603 is 3.5V, the output is 0V. Thus for a selected cell603 Vt of −3 to 3.5V, the output of the differential amplifier 606covers the entire Vt window of 6.5V to 0V.

The target cells store data as has been described. The reference cellsstore a voltage that is known when it is programmed, and can be used todetermine potential failures due to cycling and charge loss, and thelike. Sensing a difference between two cells should remove the effectsof process variations such as differences due to temperature variations,for example. Strings of reference cells and strings of target cells formpart of the differential amplifier, and a difference between selectedtarget and reference cells is obtained using the differential amplifier.Typically, a negative voltage, such as that in a range from 0 volts to−3 volts, represents a single data state (a ground potential on the wordline activates the cell regardless of where in the range the negativevoltage actually is). However, looking at a difference between a targetcell and a reference cell makes the entire range available, since it isa difference and not an absolute voltage which is the concern whensensing a difference.

FIG. 6 shows another embodiment 650 including a differential amplifier606 (such as described above), in conjunction with a memory array 601having two strings 602 and 604 of memory cells. When string 602 is aselected string for a memory operation, the cell of string 602 that isselected, cell 603 as shown, has a select voltage Vsel applied to itsgate, while other cells in the string have a pass voltage Vpass appliedto their gates. Vpass is sufficient to turn the devices on. To selectstring 602, transistors SGD and SG1 are turned on. This places string602 into the differential amplifier 606 as is shown also in FIG. 5. Inthe reference string 608, dummy reference cell 609 is coupled to nodeVout, while the remaining cells of string 5608 are coupled to passvoltage Vpass. To select string 604 as the selected string, transistorsSGD and SG2 are turned on.

If the reference string is a string of reference cells, in oneembodiment the dummy reference cell 609 coupled to Vout is in the sameposition in the reference string 608 as the selected cell 603 of theselected string 602. In other embodiments, a single reference cell isused, or a cell in a different position in the string 608 is used. Aslong as the reference string or cell varies with temperature the sameway the selected string varies, performance of the differential circuitwill be acceptable. A string of reference cells is used in oneembodiment since the string of reference cells varies more accuratelywith temperature than a single reference cell.

The various embodiments include circuits and methods for sensing againsta reference cell, that is, sensing a threshold voltage differencebetween a reference memory cell and a target memory cell. This isfacilitated by connecting a target memory cell and a reference memorycell to a differential amplifier, and sensing a threshold voltagedifference between the target memory cell and the reference memory cell.The use of a differential instead of an absolute voltage allows for theuse of a wider range of voltages than when using absolute voltages. Thevarious embodiments described with respect to FIGS. 5-6 are amenable foruse with the various memory devices and systems described above withrespect to FIGS. 1-4.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe disclosure will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the disclosure.

1. A method for expanding a usable threshold voltage range of amulti-level memory cell, comprising: sensing a threshold voltagedifference between a target cell in a string of memory cells and areference cell; wherein expanding further comprises using an absolutevoltage window including a negative voltage as a differential window. 2.The method of claim 1, wherein the target cells and the reference cellare formed at substantially the same time.
 3. The method of claim 1,wherein the target cells and the reference cell are formed usingsubstantially the same process.
 4. The method of claim 1, wherein thetarget cell is programmed to a voltage within one of at least twosub-ranges within at least a portion of a negative threshold voltagerange, wherein each of the at least two sub-ranges correspond todifferent data states.
 5. A memory device comprising: a first cell; asecond cell; and a differential amplifier configured to determine athreshold voltage difference between the first cell and the second cell.6. The memory device of claim 5, wherein a negative voltage portion of athreshold voltage range of the first cell can correspond to a pluralityof data states.
 7. The memory device of claim 5, wherein the first celland the second cell are formed at substantially the same time.
 8. Thememory device of claim 5, wherein the first cell and the second cell areformed using substantially the same process.
 9. The memory device ofclaim 5, wherein the differential amplifier comprises a set of PMOStransistors.
 10. The memory device of claim 5, wherein the first cellcomprises a selected cell and the second cell comprises a referencecell.
 11. The memory device of claim 5, wherein the first cell is one ofa first string of cells.
 12. The memory device of claim 11, wherein thesecond cell is one of a second string of cells.
 13. The memory device ofclaim 12, wherein the first cell is in the same position within thefirst string as the second cell is within the second string.
 14. Thememory device of claim 5, wherein the second cell is programmed to aknown voltage.
 15. The memory device of claim 5, wherein the first cellvaries with temperature in the same way as the second cell varies withtemperature.
 16. The memory device of claim 5, wherein the first celland the second cell are substantially similar in construction.
 17. Thememory device of claim 5, wherein the first cell is programmed to avoltage within one of at least two sub-ranges within at least a portionof a negative threshold voltage range, wherein each of the at least twosub-ranges correspond to different data states.
 18. The memory device ofclaim 5, wherein a gate of the second cell is coupled to an output ofthe differential amplifier.
 19. A differential amplifier, comprising: atarget cell; a reference cell; a current mirror load; and a bias currentsource; wherein the differential amplifier is configured to determine adifference between the first cell and the second cell.
 20. The memorydevice of claim 19, wherein a negative voltage portion of a thresholdvoltage range of the target cell can correspond to a plurality of datastates.
 21. The memory device of claim 19, wherein the target cell andthe reference cell are formed at substantially the same time.
 22. Thememory device of claim 19, wherein the targer cell and the referencecell are formed using substantially the same process.
 23. The memorydevice of claim 19, wherein the differential amplifier comprises a setof PMOS transistors.
 24. The memory device of claim 19, wherein thetarget cell comprises a selected cell.
 25. The memory device of claim19, wherein the target cell is one of a first string of cells.
 26. Thememory device of claim 25, wherein the reference cell is one of a secondstring of cells.
 27. The memory device of claim 26, wherein the targetcell is in the same position within the first string as the referencecell is within the second string.
 28. The memory device of claim 19,wherein the reference cell is programmed to a known voltage.
 29. Thememory device of claim 19, wherein the target cell varies withtemperature in the same way as the reference cell varies withtemperature.
 30. The memory device of claim 19, wherein the target celland the reference cell are substantially similar in construction. 31.The memory device of claim 19, wherein the target cell is programmed toa voltage within one of at least two sub-ranges within at least aportion of a negative threshold voltage range, wherein each of the atleast two sub-ranges correspond to different data states.
 32. The memorydevice of claim 19, wherein a gate of the reference cell is coupled toan output of the differential amplifier.